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Pnmremap User Manual(0)                                Pnmremap User Manual(0)

       pnmremap - replace colors in a PNM image with colors from another set









       All options can be abbreviated to their shortest unique prefix.  You may use two hyphens instead of one to des-
       ignate an option.  You may use either white space or an equals sign between an option name and its value.

       This program is part of Netpbm(1).

       pnmremap replaces the colors in an input image with those from a palette you  specify.   Where  colors  in  the
       input  are present in the palette, they just stay the same in the output.  But where the input contains a color
       that is not in the palette, pnmremap gives you these choices:

       ?      Choose the closest color from the palette.

       ?      Choose the first color from the palette.

       ?      Use a color specified by a command option (-missing).

       ?      Dither.  This means rather than mapping pixel by pixel, pnmremap uses colors from the palette to try  to
              make  multi-pixel  regions  of  the output have the same average color as the input (for another kind of
              dithering, see ppmdither).

       Two reasons to use this program are: 1) you want to reduce the number of colors in the input image; and 2)  you
       need to feed the image to something that can handle only certain colors.

       To reduce colors, you can generate the palette with pnmcolormap.

       By  default,  pnmremap  maps  an  input  color  that  is not in the palette to the closest color that is in the
       palette.  Closest means with the smallest cartesian distance in the red, green, blue brightness space (smallest
       sum  of the squares of the differences in red, green, and blue ITU-R Recommedation BT.709 gamma-adjusted inten-

       You can instead specify a single default color for pnmremap to use for any color in the input image that is not
       in the palette.  Use the -missing option for this.

       You  can also specify that the first color in the palette image is the default.  Use the -firstisdefault option
       for this.

       The palette is simply a PNM image.  The colors of the pixels in the image are the colors in the palette.  Where
       the  pixels  appear in the image, and the dimensions of the image, are irrelevant.  Multiple pixels of the same
       color are fine.  However, a palette image is typically a single row with one pixel per color.

       If you specify -missing, the color you so specify is in the palette in addition to whatever is in  the  palette

       For  historical  reasons,  Netpbm sometimes calls the palette a 'colormap.' But it doesn't really map anything.
       pnmremap creates its own map, based on the palette, to map colors from the input image to output colors.

   Palette/Image Type Mismatch
       In the simple case, the palette image is of the same depth (number of planes, i.e. number of components in each
       tuple (pixel)) as the input image and pnmremap just does a straightforward search of the palette for each input
       tuple (pixel).  In fact, pnmremap doesn't even care if the image is a visual image.

       But what about when the depths differ?  In that case, pnmremap converts the input image (in its own memory)  to
       match the palette and then proceeds as above.

       There are only two such cases in which pnmremap knows how to do the conversion:  when one of them is tuple type
       RGB, depth 3, and the other is tuple type GRAYSCALE or BLACKANDWHITE, depth 1; and vice versa.

       In any other case, pnmremap issues and error message and fails.

       Note that as long as your input and palette images are PNM, they'll always fall into one of the cases  pnmremap
       can handle.  There's an issue only if you're using some exotic PAM image.

       Before  Netpbm  10.27  (March  2005), pnmremap could not handle the case of a palette of greater depth than the
       input image.  (It would issue an error message and fail in that case).  You can use ppmtoppm  to  increase  the
       depth of the input image to work around this limitation.

       In any case, the output image has the same tuple type and depth as the palette image.

   Multiple Image Stream
       pnmremap  handles  a  multiple  image input stream, producing a multiple image output stream.  The input images
       need not be similar in any way.

       Before Netpbm 10.30 (October 2005), pnmremap ignored any image after the first.

       pnmcolormap testimg.ppm 256 >palette.ppm

       pnmremap -map=palette.ppm testimg.ppm >reduced_testimg.ppm

       To limit colors to a certain set, a typical example is to create an image for posting on the  World  Wide  Web,
       where  different browsers know different colors.  But all browsers are supposed to know the 216 'web safe' col-
       ors which are essentially all the colors you can represent in a PPM image with a maxval of 5.  So  you  can  do

       pamseq 3 5 >websafe.pam

       pnmremap -map=websafe.pam testimg.ppm >websafe_testimg.ppm

       Another useful palette is one for the 8 color IBM TTL color set, which you can create with
       pamseq 3 1 >ibmttl.pam

       If  you  want  to  quantize one image to use the colors in another one, just use the second one as the palette.
       You don't have to reduce it down to only one pixel of each color, just use it as is.

       The output image has the same type and maxval as the palette image.

       There is one parameter, which is required: The file specification of the input PNM file.

              This names the file that contains the palette image.

              This option is mandatory.




       -nofs  These options determine whether pnmremap does Floyd-Steinberg dithering.  Without Floyd-Steinberg,  pnm-
              remap  selects  the  output  color  of a pixel based on the color of only the corresponding input pixel.
              With Floyd-Steinberg, pnmremap considers regions of pixels such that the average color of  a  region  is
              the  same  in  the  output as in the input.  The dithering effect appears as a dot pattern up close, but
              from a distance, the dots blend so that you see more colors than are present in the color map.

              As an example, if your color map contains only black and white, and the input image has 4 adjacent  pix-
              els  of  gray,  pnmremap  with  Floyd-Steinberg would generate output pixels black, white, black, white,
              which from a distance looks gray.  But without Floyd-Steinberg, pnmremap would generate 4 white  pixels,
              white being the single-pixel approximation of gray.

              Floyd-Steinberg gives vastly better results on images where unmodified quantization has banding or other
              artifacts, especially when going to a small number of colors such as the above  IBM  set.   However,  it
              does take substantially more CPU time.

              -fs is a synomym for -floyd.  -nofs is a synonym for -nofloyd.

              The default is -nofloyd.

              Before  Netpbm  10.46  (March  2009),  dithering doesn't work quite as you expect if the color map has a
              lower maxval than the input.  pnmremap reduces the color resolution to the  color  map's  maxval  before
              doing  any  dithering,  so  the  dithering  does not have the effect of making the image, at a distance,
              appear to have the original maxval.  In current Netpbm, it does.

              This option affects a detail of the Floyd-Steinberg dithering process.  It has no effect if  you  aren't
              doing Floyd-Steinberg dithering.

              By default, pnmremap initializes the error propagation accumulator to random values to avoid the appear-
              ance of unwanted patterns.  This is an extension of the original Floyd-Steinberg algorithm.

              A drawback of this is that the same pnmremap on the same input produces slightly different output  every
              time, which makes comparison difficult.

              With  -norandom,  pnmremap  initializes the error accumulators to zero and the output is completely pre-

              -norandom was new in Netpbm 10.39 (June 2007).

              This tells pnmremap to map any input color that is not in the palette to the first color in the  palette
              (the color of the pixel in the top left corner of the palette image)

              See DESCRIPTION .

              If you specify -firstisdefault, the maxval of your input must match the maxval of your palette image.

              This  specifies  the  default  color for pnmremap to map to a color in the input image that isn't in the
              palette.  color may or may not be in the palette image; it is part of the palette regardless.

              If you specify -missingcolor, the maxval of your input must match the maxval of your palette image.

              Display helpful messages about the mapping process.

       pnmcolormap(1), pamseq(1), pnmquant(1), ppmquantall(1), pamdepth(1), ppmdither(1), ppmquant(1), ppm(1)

       pnmremap first appeared in Netpbm 9.23 (January 2002).  Before that, its function was available only as part of
       the  function  of pnmquant (which was derived from the much older ppmquant).  Color quantization really has two
       main subfunctions, so Netpbm 9.23 split it out into two separate programs: pnmcolormap and  pnmremap  and  then
       Netpbm 9.24 replaced pnmquant with a program that simply calls pnmcolormap and pnmremap.

       Copyright (C) 1989, 1991 by Jef Poskanzer.

netpbm documentation             03 June 2009          Pnmremap User Manual(0)